$75 (or best offer): I have 2 twin-sized (38" by 75") 11" thick Tempur-Pedic mattresses. This is the brand that has set the standard for high quality memory foam mattresses.
The main benefit of the Tempur-Pedic mattress is pressure distribution. Instead of pushing back mainly at the heaviest contact points — hips, shoulders, heels — the foam gradually conforms around the body. That can make the mattress feel less like a flat surface and more like a molded support cradle.
So, it conforms to body weight, shape, and temperature, which can reduce pressure at the shoulders, hips, back, and joints, unlike a spring mattress that pushes back in a more generalized way.
This can be helpful for side sleepers and some back sleepers. Tempur-Pedic is usually more expensive, but part of what buyers are paying for is denser, higher-quality foam that can retain its feel longer than many budget foam mattresses.
A cheaper memory-foam mattress may feel similar at first but may use lower-density foams that soften, sag, or develop body impressions sooner.
New ones starting at $1,599 on Amazon are warrantied for 10 years. These two are 3 years old and in very good condition.